fastness vs stronghold

fastness and stronghold both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastness	Yes	No	No	No
stronghold	Yes	No	No	No

As nouns, fastness and stronghold are synonyms defined as:

fastness (noun)	stronghold (noun)
a strongly fortified defensive structure	a strongly fortified defensive structure
a rate (usually rapid) at which something happens
the quality of being fixed in place as by some firm attachment